The balance in normal subjects between on the one hand bone formation, which is associated with the number and activity of osteoblasts, that is cells associated with the production of bone in the organism, and on the other hand bone loss, which is associated with the number and activity of osteoclasts, that is cells associated with the absorption and removal of bone, is disturbed in several bone affecting diseases. At the present time there is no good treatment for any of these diseases, among which can be mentioned osteoporosis, Paget's disease of bone, hyperparathyroidism and related disorders, and several malignant neoplasms where tumor cells are producing osteoclast-activating factors and cause hypercalcemia.
Worldwide the most urgent need is for the treatment of osteoporosis and tumor associated hypercalcemia. In some areas, e.g. in England and in some other parts of Europe there is also high incidence of Paget's disease of bone.
In osteoporosis bone formation as well as bone resorption are disturbed, resulting in decreased bone mass. Osteoporosis predominantly affects the elderly, but also other groups such as postmenopausal women, where an estrogen deficit is believed to be a significant etiological factor, and immobilized patients. At this point it is not possible to clear up the whole picture of the disease mechanism and estimate which is the primary cause of osteoporosis. However, about 25% of osteoporotic females belong to what is called "rapid bone losers" and at least in those patients the bone resorption rate is probably increased. Landry and Fleisch showed in immobilization induced osteoporosis that bone resorption rate was accelerated, (Landry, M. and Fleisch, H,: The influence of immobilization on bone formation as evaluated the incorporation of tetracyclines. J. Bone Joint Surg. 46B: 764, 1964).
The clinical manifestations of osteoporosis comprise fractures, especially hip fractures, but also vertebral fractures and fractures of the proximal radius, and complication of such fractures.
In Finland it has been estimated that about 10% of all surgical hospital beds are used for the treatment of osteoporosis related fractures (Luthje, P.: Reisiluunkaulan ja trokantterin murtumapotilaiden hoito ja ennuste seka hiodon kustannukset. Thesis. Helsinki 1983).